An optical modulator is a device which can be used for manipulating a property of light, often of an optical beam, e.g. a laser beam. Depending on which property of light is controlled, modulators may be intensity modulators, phase modulators, polarization modulators, spatial light modulators, and so forth. A wide range of optical modulators are used in very different application areas, such as in optical fiber communications, displays, mode locking of lasers, and the like.
An electroabsorption modulator is a device which can be used for modulating the intensity of a light beam in response to an applied voltage. Such devices are often useful for optical communications. As is well known in the art, carriers are generated by the optical absorption of photons in the modulator. More specifically, a photon that has an energy corresponding to a wavelength that is near or above the bandgap energy, which is also referred to as the bandgap wavelength, will be absorbed. Typically such absorption will result in the generation of an electron-hole pair.
Once a certain density of electron-hole pairs is reached, the electron-hole pairs will start to screen the externally applied electric field, in that the buildup of electrons and holes in a particular location reduces the strength of the externally applied field. Unfortunately, this has the detrimental effect on modulator performance of lowering the modulation bandwidth, i.e., the rate at which the modulator can be changed from a transmissive state that allows a prescribed amount of the light through, to a blocking state and back. This is due to higher capacitance resulting from the high carrier concentrations.
Prior art electroabsorption modulators typically incorporate a single bandgap along the optical path. Disadvantageously, this limits the optical power-handling ability of such an electroabsorption modulator due to the aforementioned carrier screening effects. Additionally, the modulation bandwidth of the prior art electroabsorption modulators is less than ideal for high-input optical power applications, and the device could be damaged if fed with high optical power.